Design, Synthesis and Aromaticity of an Alternating Cyclo[4]Thiophene[4]Furan.

A new class of conjugated macrocycle, the cyclo[4]thiophene[4]furan hexyl ester (C4TE4FE), is reported. This cycle consists of alternating α-linked thiophene-3-ester and furan-3-ester repeat units, and was prepared in a single step using Suzuki-Miyaura cross-coupling of a 2-(thiophen-2-yl)furan monomer. The ester side groups help promote a syn conformation of the heterocycles, which enables formation of the macrocycle. Cyclic voltammetry studies revealed that C4TE4FE could undergo multiple oxidations, so treatment with SbCl5 resulted in formation of the [C4TE4FE]2+ dication. Computational work, paired with 1 H NMR spectroscopy of the dication, revealed that the cycle becomes globally aromatic upon 2e- oxidation, as the annulene pathway along the outer ring becomes Hückel aromatic. The change in ring current for the cycle upon oxidation was clear from 1 H NMR spectroscopy, as the protons of the thiophene and furan rings shifted downfield by nearly 6 ppm. This work highlights the potential of sequence control in furan-based macrocycles to tune electronic properties.

Porphyrin/Quinoidal-Bithiophene-Based Macrocycles and Their Dications: Template-Free Synthesis and Global Aromaticity.

We report the template-free synthesis and characterization of a new type of porphyrin/quinoidal-bithiophene-based conjugated macrocycle. X-ray crystallographic analysis of the dimer (2MC) revealed a cyclophane-like geometry with large dihedral angles between the porphyrin and the neighboring thiophene rings, and NMR measurements and theoretical calculations confirmed a localized aromatic character of the porphyrin/thiophene rings and quinoidal character of the bithiophene linkers. Restricted rotation of the thiophene rings linked to the porphyrin unit was observed by variable-temperature NMR measurements. The dication (2MC2+ ) adopts a chair-shaped conformation to facilitate π-electron delocalization around the whole macrocycle. As a result, the molecule is globally aromatic, with a dominant 54 π conjugation pathway. The trimer (3MC) also shows localized aromatic character of porphyrin rings and conformational flexibility, but its dication (3MC2+ ) is rigid and globally aromatic with a dominant 82 π conjugation pathway.

A Stable All-Thiophene-Based Core-Modified [38]Octaphyrin Diradicaloid: Conformation and Aromaticity Switch at Different Oxidation States.

A soluble and stable core-modified [38]octaphyrin, MC-T8, containing eight thiophene rings was synthesized by Yamamoto coupling followed by oxidative dehydrogenation. X-ray crystallographic analysis revealed a nearly planar backbone, and the molecule is globally aromatic with a dominant 38π conjugation pathway. The dication MC-T82+ is antiaromatic, and the backbone is distorted, with a different orientation of the thiophene rings. The tetracation MC-T84+ becomes aromatic again, with a shallow-bowl-shaped geometry. Both the neutral compound and the dication demonstrated open-shell diradical character with a small singlet-triplet energy gap (-2.70 kcal mol-1 for MC-T8 and -3.78 kcal mol-1 for MC-T82+ ), and they are stable owing to effective spin delocalization.

Global Aromaticity in Macrocyclic Cyclopenta-Fused Tetraphenanthrenylene Tetraradicaloid and Its Charged Species.

A stable cyclopenta-fused tetraphenanthrenylene macrocycle, CPTP-M, was synthesized, and the structure was confirmed by X-ray crystallographic analysis. It exhibits a large radical character (number of unpaired electron, NU =3.52) and a small singlet-triplet energy gap (ΔES-T =-2.8 kcal mol-1 by SQUID). Its backbone contains 60 ([4n]) conjugated π electrons and is globally antiaromatic. NMR measurements and theoretical calculations revealed that its dication/dianion is globally aromatic owing to the existence of [4n-2]/[4n+2] π-conjugated electrons. Remarkably, the ring-current map of the tetraanion shows a unique ring-in-ring structure, with a diamagnetic outer ring and a paramagnetic inner ring. Accordingly, both the inner-rim and outer-rim protons are deshielded in its 1 H NMR spectrum. The tetraanion can be regarded as an isoelectronic structure of the known octulene, which shows similar electronic properties.